1. Field of the Invention
The present invention relates to a method for uniformly coating a conductive film on a panel surface. The invention is particularly advantageous in providing an electrically conductive film on the panel surface of a cathode ray tube.
2. Description of the Prior Art
Generally, cathode ray tubes and liquid crystal displays have a front panel made of an insulating material such as glass or plastic. As a result, an electrostatic force occurs at the surface of front panel, thereby attracting dust to the surface. In particular, when such an electrostatic force occurs at the front surface of a cathode ray tube in a television receiver or in other electric displays, it may cause the displays to malfunction. In severe cases, the displays may be permanently damaged.
In order to solve these problems, various methods have been proposed for providing a cathode ray tube having an electrically conductive surface, thereby preventing the cathode ray tube from being electrostatically charged. Among these methods, is a method for coating a metal film or a conductive glass oxide film on the surface of a cathode ray tube using a deposition technique or other techniques.
A method of forming a conductive film on the surface of a cathode ray tube is disclosed in Japanese Patent Laid-open Publication No. 100943/1982. This method includes the following steps:
a) cleaning a front panel of the cathode ray tube with an abrasive and a cleaning agent;
b) pre-heating the surface of the front panel in a furnace to a temperature of about 60.degree. C. and then coating a liquid conductive film-forming coating material over the panel surface using a spray nozzle (such a coating material is disclosed in PCT Publication No. WO 88/06331 published on Aug. 25, 1988); and
c) baking the coating material on the panel surface in a furnace at a temperature of about 150.degree. C. for about 30 minutes, so as to crystalize the coating material.
The film obtained by crystallizing the coating material on the panel surface provides desirable characteristics such as electric surface resistance, brightness, roughness and film strength.
Accordingly, to uniformly coat the coating material over the panel surface of cathode ray tube is a very important factor in obtaining desirable panel surface characteristics in a cathode ray tube.
Generally, coating the coating material over the panel surface of a cathode ray tube is carried out, not manually, but automatically by moving a nozzle along a predetermined path using an automatic device such as a robot. A conventional method for coating a coating material over the panel surface of a cathode ray tube is carried out according to the procedures illustrated in FIG. 1.
In accordance with the conventional method, a spray nozzle moves from a point X to a point Y in a zig-zag manner along a path on a cathode ray tube panel 1. From the point Y, the spray nozzle then moves to the original point X in a zig-zag manner along a path crossing the previous path. As the spray nozzle moves along its path, a coating material is spread over the panel surface of the cathode ray tube at an inclined angle of travel.
The spray nozzle travels from the point X to the point Y along a path as indicated by the solid line in FIG. 1. The nozzle then returns to the point X along a path indicated by the dotted line in FIG. 1. The nozzle is held in a vertical position and moves along the curvature of the panel surface during spraying, as indicated by the arrows in FIG. 2.
In the conventional method, however, the thickness of the coating material becomes irregular throughout the panel surface of the cathode ray tube, since the nozzle coats the coating material while moving along at an inclined angle of travel. The coated film is relatively thick at areas "A" where the inclined travel paths cross each other, and the amount of coating material coated at the areas "A" is considered to be excessive. The areas "B", which correspond to portions of the inclined travel paths in which there was no crossover, are relatively thin. This irregular coating thickness forms surface irregularities which result in irregularity of electric surface resistance and brightness at both areas A and B.
Table 1 shows electric surface resistance and brightness measured at areas A and B.
TABLE 1 ______________________________________ Area "A" Area "B" ______________________________________ Electric Surface 6.16 .times. 10.sup.10 2.0 .times. 10.sup.10 Resistance (.OMEGA./sq) Brightness (%) 56 67 ______________________________________
Since the nozzle sprays the coating material in a downward vertical direction as it moves along the curvature of the cathode ray tube, there is a difference in the distances between the nozzle and each panel surface portion located within the spraying area of the nozzle. For example, portions P1-P3 of panel surface shown in FIG. 2 have different distances from the nozzle, i.e., L1-L3, respectively. Such difference in distances becomes a factor in the irregularity of coating thickness.